Disk drives store information on magnetic disks. Typically, the information is stored in concentric data tracks on the disks. The tracks are usually divided into sectors. Information is written to and read from the disk by a transducer head. The transducer head may include a read head separate from a write head, or the read and write heads may be integrated. The transducer head is mounted on an actuator arm assembly that moves the transducer head radially over the disk. Accordingly, the movement of the actuator arm assembly allows the transducer head to access different tracks. The disk is rotated by a spindle motor at a high speed, allowing the transducer head to access different sectors within each track on the disk.
The actuator arm assembly is interconnected to a motor, such as a voice coil motor, to move the transducer head radially over the disk. The voice coil motor is controlled by a servo control system. The servo control system performs two functions: seeking and track-following. The seek function moves the transducer head from an initial position to a target track. The seek function is initiated when a host computer associated with the disk drive issues a command to read data from or write data to the target track. Because of increasingly high demands on the performance of computer storage devices such as disk drives, it is desirable to move the transducer head from its initial position to the target track as quickly as possible. Once the transducer head is sufficiently close to the target track, the track-following function is activated to center and maintain the transducer head on the target track until the desired data transfer is complete.
The transducer head will oscillate about the centerline of the target track for a time period following the transition from the seek mode to the track-following mode. Because data written while the transducer head is oscillating about the centerline of the target track may be unrecoverable during subsequent attempts to read the data, the write operations are prohibited for a time period following the transition from the seek mode to the track-following mode. In addition, because data from adjacent tracks may inadvertently be read, or may corrupt the read signal generated by the transducer head during a read operation attempted while the transducer head is oscillating, the read operations are also inhibited for a time period following the transition from the seek mode to the track-following mode. Settle time during which reading and writing by the transducer head is not allowed better ensures the integrity of data written to or read from the disk.
Disk drives are susceptible to data errors due to external shock events. This is because shock can cause the transducer head to deviate from a desired position over the centerline of a track. Therefore, it is important to prohibit the transfer of data to and from the disk during shock events. In particular, it is important to prohibit writing data to the disk when shock events occur to prevent unrecoverable errors when the data is written to unintended areas of the disk.
The “centerline” of the track does not necessarily coincide with the physical centerline of the track. Instead, the “centerline” may refer to the center of the intended data storage area of the track. Therefore, as used herein, the “centerline” of a track need not refer to the physical centerline of the track, and “centered” indicates that the transducer head is properly centered over the area within the track that is intended for data storage, regardless of whether that position coincides with the physical centerline of the track.
Track misregistration error occurs when the transducer head is not properly centered over the track. Read track misregistration error occurs when the read head of the transducer head is not properly centered over the track centerline. Likewise, write track misregistration error occurs when the write head of the transducer head is not properly centered over the track centerline. Write track misregistration errors are particularly troublesome because they can result in permanent data loss. For instance, data written while the write head is not centered over the track may be unrecoverable during subsequent read operations because the read head, traveling over the track centerline and looking for the data in the expected position, may not be able to retrieve the data written off-center. In addition, data written to adjacent tracks may be lost since data written off-center may overwrite or corrupt data in an adjacent track. Thus, it is important to detect shock events and prohibit writing while the transducer head is not properly centered over the target track.
A write fault occurs when the transducer head deviates a predetermined distance from the centerline of the target track and the servo control system is in the track-following mode. The disk drive may trigger a write fault in response to a shock event and maintain the write fault for a predetermined time period to allow oscillations caused by the shock event to dampen and disappear. While the write fault is in effect, the write operations are disabled.
In setting the distance that the transducer head must deviate from the track centerline (the magnitude of the tracking error) to trigger the write fault, and in setting the time during which the write operations are prohibited, consideration must be given to the data transfer rate of the disk drive. Delaying the write operations avoids track misregistration errors but also reduces the data transfer rate. Although the data transfer rate is of great concern, the integrity of the data is of paramount importance.
Previous methods of detecting shock events and triggering write faults use accelerometers and other devices that are not required for the basic functions of the disk drive. Accordingly, previous shock detection methods increase the cost of the disk drive. In addition, previous shock detection methods treat all shock events equally, regardless of the severity of the shock event, and therefore unnecessarily compromise the data transfer rate and data loss resistance of the disk drive.
It would be advantageous to provide a disk drive that reacts to shock events in different ways, depending on the severity of a particular shock event. In addition, it would be advantageous to provide a disk drive that registers the severity of shock events without a separate shock detector. Furthermore, it would be advantageous to provide a disk drive that protects against track misregistration errors without unduly limiting the data transfer rate.